The trades are sent to the blockchain by our servers. In order to allow for a fast and smooth execution of orders our servers send the transactions with your signature to the blockchain.
This is done so there are no competing trades for the same order and allows us to offer you the user experience of a centralized exchange while having the safety of a decentralized one. Therefore the gas fee for the transaction is paid by us and deducted from your balance during the trade execution. We set the gas fee depending on the network congestion so that the transactions are executed as fast as possible. You can see the estimated gas fee when you place the order.